Capacitor backup for SRAM

ABSTRACT

Embodiments of the disclosure provide a method for backing up data in an SRAM device, and an SRAM device that includes a capacitive backup circuit for backing up data in an SRAM device. The method may include writing data to the SRAM cell by applying an input voltage to set an input node of cross-coupled inverters to a memory state. The method may also include backing up the data written to the SRAM cell by electrically coupling the input node to the capacitive backup circuit. The method may also include restoring the data stored in the capacitive backup circuit to the SRAM cell by electrically coupling the capacitive backup circuit to the input node.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/912,324, filed Jun. 7, 2013. The aforementioned relatedpatent application is herein incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a static random-access memory (SRAM).In particular, this disclosure relates to backing up and restoring oneor more SRAM cells.

BACKGROUND

SRAM memory cells are fast and dense storage devices. The size anddensity of the SRAM memory cells may make them susceptible to softerrors, which can be caused by radiation spikes from alpha particles,beta particles, gamma radiation, or ionized metal particles, which maychange the state of the SRAM. These soft errors may cause errors thatmust be caught and corrected in order for the computer to continuefunctioning correctly.

SUMMARY

Embodiments of the disclosure provide a method for backing up data in anSRAM, and an SRAM device that includes a capacitive backup circuit.

One embodiment is directed towards a method of backing up an SRAM cellto a capacitive backup circuit of the SRAM cell. The method may includewriting data to the SRAM cell by applying an input voltage to set aninput node of cross-coupled inverters to a memory state. The method mayalso include backing up the data written to the SRAM cell byelectrically coupling the input node to the capacitive backup circuit.The method may also include restoring the data stored in the capacitivebackup circuit to the SRAM cell by electrically coupling the capacitivebackup circuit to the input node.

Another embodiment is directed towards a Static Random Access Memory(SRAM) device. The SRAM device may include one or more SRAM cells. EachSRAM cell may have a first and a second CMOS inverter that arecross-coupled. The SRAM cell may also have a feedback switch coupled toan output of the first CMOS inverter and an input of the second CMOSinverter. The SRAM cell may also have a capacitive backup circuitcoupled to the output of the feedback switch. The capacitive backupcircuit may have a capacitor and a backup switch configured in a seriesconnection between the capacitor and the feedback switch.

Another embodiment is directed towards an SRAM device. The SRAM devicemay include one or more SRAM cells. Each SRAM cell may havecross-coupled inverters that are configured and arranged to hold amemory state. Each SRAM cell may also have a capacitive backup circuithaving at least one capacitive element. The capacitive backup circuitmay be configured and arranged to store a backup copy of the memorystate in response to the deactivation of a feedback switch and theactivation of an input voltage. The capacitive backup circuit may alsobe configured to provide a backup copy of the memory state in responseto the deactivation of the feedback switch and the deactivation of theinput voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings included in the present application are incorporated into,and form part of, the specification. They illustrate embodiments of thepresent invention and, along with the description, serve to explain theprinciples of the invention. The drawings are only illustrative ofembodiments of the invention and do not limit the invention.

FIG. 1 illustrates an SRAM device that incorporates an eDRAM trenchcapacitor, according to various embodiments.

FIG. 2 illustrates an SRAM array configuration, according to variousembodiments.

FIG. 3 illustrates a timing diagram of a backup operation of the SRAMcell, according to various embodiments.

FIG. 4 illustrates a timing diagram of the normal write operation forthe SRAM device, according to various embodiments.

FIG. 5 illustrates a timing diagram 500 of the restore operation of theSRAM device, according to various embodiments.

FIG. 6 illustrates multiple design structures including an input designstructure that is preferably processed by a design process.

FIG. 7A illustrates a circuit that produces a pulse during the normalwrite operation and a corresponding waveform diagram produced by thecircuit, according to various embodiments.

FIG. 7B illustrates a circuit that produces a pulse during the restoreoperation and the corresponding waveform diagram, according to variousembodiments.

FIG. 7C illustrates a circuit that receives an input signal from a delaycircuit and an input signal from a pulse generator, according to variousembodiments.

In the drawings and the Detailed Description, like numbers generallyrefer to like components, parts, steps, and processes.

DETAILED DESCRIPTION

Aspects of the present disclosure are directed toward a method ofbacking up an SRAM cell memory state to a capacitive backup circuit ofthe SRAM cell by writing data to the SRAM cell, backing up the datawritten to the SRAM cell in the capacitive backup circuit, and restoringthe data stored in the capacitive backup circuit. Although notnecessarily limited thereto, embodiments of the present disclosure canbe appreciated in the context of integrated circuits and problemsrelating to density and scaling.

Since the development of integrated circuits, the potential for reducedsignal propagation time and increased functionality of individual chipsas well as manufacturing economy have driven the development ofincreases in integration density and scaling of individual circuitelements therein to smaller sizes. This incentive for scaling isparticularly strong for memory devices which have approximatelyquadrupled in capacity every three years. However, such scaling ofindividual elements inevitably increases criticality of many aspects ofoperation such as operation at reduced voltages, susceptibility tonoise, heat dissipation and the like.

Among other problems aggravated by scaling, particularly in memorystructures, is the problem of soft errors due to the transit ofradiation through a memory cell. Radiation is substantially unavoidablein the environment and its relatively high energy causes ionizationwhich can introduce charge into the circuit in an unpredictable mannerbut without causing damage to the circuit element or impairing itsfunction. This is particularly true in deep space applications such assatellite circuitry. In practical effect in a memory cell, thedisturbance of charge may be sufficiently great that the storage orlogic state of the cell will be changed and the data corrupted but thecell will be capable of functioning correctly when new data is stored.For this reason, such errors are referred to as “soft” as opposed to“hard” errors such as a failed transistor or a wiring defect whichprevents correct functioning of the circuit.

Scaling causes increased susceptibility to soft errors because itdecreases the amount of charge required to induce a soft error through anumber of mechanisms. That is, the amount of charge developed by thetraverse of a given radiation particle through a circuit is highlyvariable depending on a number of factors such as the energy of theradiation, its trajectory through the circuit and the like. Therefore,not all incident radiation will induce a soft error. However, reductionof the amount of charge necessary to induce a soft error can beconsidered as a reduction in the stability of a memory cell and, for agiven distribution of energies and trajectories of a given flux ofradiation, the number of soft errors will increase as the criticalamount of charge that will induce a soft error, Q_(crit), decreases. Theamount of charge that can upset a memory cell of a given type has beendecreasing by about one-half per generation; more or less proportionallyto the decrease in circuit element and memory cell dimensions.

Further, numbers of soft errors per chip are increased by increasedintegration density. Substantially increased total soft error rates ofabout 50K ppm/khr/chip (with some significant differences between low tohigh and high to low transition errors) are projected for 8 Megabytememories currently being designed and manufactured having a cell size ofabout 2.5 to 5 μm². That is, while soft errors are relatively rare andcurrent memory cell designs are highly reliable and stable, such amemory can now be expected to experience a soft error, on average, everyone to two months and may vary significantly (e.g. may double or more)with the logic state stored. (By way of comparison, soft error rateswere negligible in SRAM designs of only a few generations previous tocurrent designs.) Given the amount of memory generally associated withmodern processors and the fact that any soft error corrupts data or anapplication program, it can be readily appreciated that some provisionmust be made for correction or avoidance of soft errors in order tosupport acceptable processor performance.

It should also be appreciated that many different types of data storagestructures are known in the art; each with its own potentialities andproblems. For example, so-called SRAMs offer the fastest access timebecause they need not be refreshed or use sense amplifiers capable ofsensing a minute difference in charge which may slow response time indynamic memories. Thus static memories are often used for caching at alocation hierarchically adjacent to a processor, often on the same chipwith the processor or other logic devices such as in registers.

However, static memory cells present problems which have been largelyintractable in regard to soft errors, particularly as attempts are madeto increase chip capacity. Specifically, static memory cells requiremore active devices (e.g. transistors) per memory cell and thereforescaling must be more aggressive to increase memory capacity andintegration density. At the same time, scaling decreases the currentcarrying capacity of individual transistors and can reduce stability ofthe cell and well as the ability of the cell to drive the capacitance ofconnected bit lines.

Additionally, since static memories are hierarchically proximate to thehigh speed circuitry which utilizes the data stored, the opportunity forproviding error detection and correction is reduced and error checkingand correction often compromises response and memory access speed,particularly if correction must be performed. Therefore, the problem ofsoft errors is particularly critical in SRAM structures and it isanticipated that the invention will have most beneficial applicabilityin that environment. Accordingly, while the invention will be disclosedin connection with an SRAM environment which is currently preferred bythe inventors, it should be understood that the invention is applicableto other integrated circuit environments, as well.

Traditionally, error correction code on a system on a chip may belimited in the number of errors that it can detect and fix. During asoft error, another response for the system is to reboot which costsdown time. In environments with periodically high radiation spikes,e.g., deep space, it may be especially advantageous to restore an SRAMto a known state prior to a data corruption event, e.g., a radiationspike. Furthermore, in any SRAM environment, error correction typicallyis capable of correcting a single bit error and detecting a double biterror. A word in a capacitor-backed SRAM may be capable of beingcorrected by using the value stored in the capacitor, even if the errorchecking logic, such as ECC (error checking and correcting) circuitry,detects a multibit error.

An aspect of this disclosure is to include an eDRAM trench capacitor ineach SRAM cell of an SRAM device, along with the gates to control accessto the capacitor. For example, one or more n-type Field EffectTransistors (NFETs) may be used to control access to the eDRAM trenchcapacitor or isolate the capacitor. The SRAM device may perform a writeor backup operation to record a state of the SRAM cell in a capacitivebackup circuit. The saved state can then be read when restoring the dataonce cell corruption is determined. The capacitive backup circuit canuse a capacitive element to store the state and may be referred to as acapacitive backup circuit through this disclosure. To compensate forleakage current, the stored state can be refreshed by performingrepeated backup operations.

The backup operation of the SRAM cell may use any number of timingsignals. The timing signals may be controlled by a memory controller insome embodiments. The memory controller may work in conjunction with acircuit designed to detect sources of soft errors, such as a radiationdetector. The radiation detector may determine high instances ofradiation. In certain embodiments, a soft error detector can be designedusing dedicated SRAM cells linked to a number of fuses programmed intothe SRAM cells. Data is written to the SRAM cells according to apredetermined signature. The fuses are each also programmed with theknown signature. The stored data from the SRAM cells may be read andcompared with the known signature on the fuses to determine if the datastored in the SRAM has been corrupted.

In some embodiments, the memory controller may adjust the frequency ofthe backup operation of the state of the SRAM cell to a capacitor on theSRAM cell based on the radiation detected. For example, if the radiationdetector detects a period of high radiation, then the memory controllermay direct the backup operation of the state of the SRAM cell to occurless frequently, e.g., every 1000 cycles instead of every 100 cycles, toreduce the likelihood of saving data that is corrupted by radiation. Inother embodiments, the radiation detector may incorporate errorcorrecting code into the SRAM cell 102 in the presence of a radiationspike to ensure that data written to the backup capacitor is correct.

The present disclosure may allow an SRAM device to operate in multipleoperational modes. For example, the SRAM device may have a normaloperation, where the SRAM device saves data in a short-termconfiguration. The SRAM device may have a backup operation, where theSRAM device records a memory state of the device to a capacitive backupcircuit. The SRAM device may also have a restore operation, where theSRAM device reads the data from the capacitive backup circuit.

FIG. 1 illustrates an SRAM device 100 that incorporates the eDRAM trenchcapacitor, according to various embodiments. The SRAM device 100 mayhave one or more SRAM cells with each SRAM cell connected to one or morewrite bit lines (WBL), e.g., WBL CMP 112. In the shown embodiment, theSRAM device 100 is configured in a single-ended write configuration witha single WBL, but a double-ended write configuration with two WBLs ispossible.

The WBLs may be in a global configuration. The WBL CMP 112 may couple toa WBL driver 113. The WBL driver 113 may drive a logical one or zerofrom a register (not pictured) to the SRAM cell 102 through the WBL CMP112. In the configuration shown, the WBL driver 113 may perform writingoperations. For example, the write and read operations are performed inseparate operations and the WBL driver 113 may perform the writeoperation while a read circuit performs the read operation.

However, SRAM device 100 may be configured to perform read and writeoperations through the WBL CMP 112. Additional components may berequired. For example, in the read operation, additional capacitors maybe needed between the pass transistor 121 and the WBL 112 in order tostabilize the read operation. When the read and write operations areboth performed through the WBL 112, then the WBL driver 113 may furtherconnect to a sense amplifier (not pictured). In the described examplewhere the SRAM device 100 performs both read and write operations, theWBL driver 113 may be tri-stated in a read operation and activated inthe write operation.

For purposes of illustration, the operation of one SRAM cell, SRAM cell102 will be described in detail. SRAM cell 102 may have twocross-coupled CMOS inverters, inverter 114 and inverter 116. In someembodiments, the inverter 114 may be referred to as a first inverterwhile the inverter 116 may be referred to as a second inverter. Thefirst inverter 114 may output to a complement (CMP) node of the SRAMcell 102, while the second inverter 116 may output to a true (TRU) nodeof the SRAM cell 102.

The SRAM cell 102 may receive an input voltage from the WBL CMP 112 atthe CMP node. The CMP node may also be referred to as an input nodethroughout this disclosure. In an embodiment, the second inverter 116may receive the input voltage and invert the voltage to form an input tothe first inverter 114. The SRAM cell 102 may hold a memory state aslong as the power to the SRAM cell 102 is active. For example, if theinput voltage is a logical one at the input node, then the secondinverter 116 may invert the input voltage to a logical zero. The memorystate may also be referred to as a charge state throughout thisdisclosure. The first inverter 114 may receive the logical zero inputfrom the second inverter 116 and output a logical one to the secondinverter 116. The memory state may refer to the value “saved” on theSRAM cell 102, which, in the mentioned example, is a logical zero at theTRU node.

The inverter 114 may invert the signal from the true (TRU) side andoutput to the complement (CMP) side through an n-channel field effecttransistor (NFET) 117 referred to as a feedback transistor or feedbackswitch 117. Through this disclosure, it may be understood that atransistor is a type of switch and the two terms may be usedinterchangeably. In one embodiment, the feedback transistor 117 canisolate the CMP node from the inverter 114. When the feedback transistor117 receives a controlling feedback signal from the feedback line (FL)118 indicating a logical high or turned “on,” the feedback transistor117 is enabled and the inverter 114 can drive an output to the CMP. Whenthe feedback line 118 is a logical low or turned “off,” the CMP node isisolated from inverter 114. The inverter 116 is configured to drive theinverse of the signal from the CMP side to the TRU side.

The CMP node of the SRAM cell 102, in an embodiment, may be connected tothe WBL CMP 112, through pass transistor 121. The pass transistor 121,may allow the coupling between the WBL 112 to CMP of the SRAM cell 102.The pass transistor 121, may also be referred to as a pass switch. Thepass transistor 121 may be selected by a write word line (WWL) 120. TheWWL 120 may be a word line that connects one or more SRAM cells in arow. The WWL 120 may be activated to write a row of SRAM cells. The SRAMcell 102 may have another pass transistor 121 that is selected by WWLCMP 120. Pass transistor 121 may connect to the WBL CMP 112. The passtransistor 121 may receive a write signal from WBL CMP 112. A logicalone signal on the WWL 120 allows access to the SRAM cell 102 through WBLCMP 112.

Consistent with various embodiments, the SRAM cell 102 can (optionally)include a read circuit 124. The read circuit 124 may be furtherconnected to domino logic or other logic that causes the leastdisturbance to a read operation of the SRAM cell 102.

The read circuit 124 may have two NFETs, N1, and N2. N1 may connect to aground at the source and to N2 at the drain. N1 may be controlled by asignal from the TRU node of the cross-coupled inverters. The readcircuit 124 may connect N1 to a read bit line (RBL) 122. N1 may becontrolled by a signal from the read word line (RWL) 123. RWL 123 may bea word line that controls the read of a row of SRAM cells. An on signalfrom the RWL 123 may activate the read of the SRAM cell 102. RBL 122 maybe a global bit line that reads one or more SRAM cells. The RBL 122 maybe pre-charged and may output to an inverter (not shown).

The read circuit 124 may be necessary in embodiments of the disclosureto allow for an asymmetric configuration of the SRAM cell 102. In theasymmetric configuration, the inverter 114 may be stronger and lesssensitive than the inverter 116 (discussed further below). By connectingthe read circuit 124 to the output of the inverter 116, the currentstrength from inverter 116 may be impacted to a lesser degree than otherconfigurations. In addition, the read circuit 124 may allow forincreased speeds relative to performing the read and write operations onthe word bit line 112.

The read circuit 124 may read the state of the SRAM cell 102 and may befurther transmitted to a register (not pictured). In the shownembodiment, the read circuit 124 is connected to the TRU node, but otherconfigurations are contemplated. The read circuit 124 may perform areading function. The reading function may read the state of the SRAMcell 102. The read circuit 124 may perform a single-ended read which mayminimize the current leakage from the SRAM cell 102 in some embodiments.The presence of the read circuit 124 may allow design flexibility foreither a symmetrical or asymmetrical SRAM cell 102.

The SRAM cell 102 may include a capacitive backup circuit 125. Thecapacitive backup circuit 125 may include a capacitor 126 and a backuptransistor 127. The backup transistor 127 may also be referred tointerchangeably with a backup switch throughout this disclosure. Thecapacitor 126 may store a signal from the SRAM cell 102 while the SRAMcell 102 is operating. In some embodiments, the capacitor 126 may beconnected in series to a resistor 129. The resistor 129 may be 2e³ ohms,or a 2K resistor. The resistor 129 may consist of any polysiliconmaterial, according to various embodiments. In certain embodiments, thecapacitor 126 may be a 22 nm deep trench metal-insulator-metal (MIM)capacitor and may include a resistance barrier. In a possibleconfiguration, the capacitor 126 may hold a charge of 1e⁻¹²C-1e⁻¹⁵ C.Consistent with various embodiments, the capacitor 126 can be designedwith sufficient voltage rating (e.g., size) to accommodate the voltagefrom the WBL 112 without damaging the capacitor 126.

Aspects of the present disclosure are based upon the recognition that astorage capacitance can be more resistant to radiation (relative to anSRAM cell) because a high energy particle is not as likely to change thestate stored on a capacitor (e.g., because the capacitor is not assusceptible to the creation of an ionization track to the substrate).

The backup transistor 127 may be an NFET, and may control access to thecapacitor 126 and allow the capacitor 126 to be isolated from the datastored in SRAM cell 102. The gating on the backup transistor 127 mayreceive a backup signal 128 that may allow a discharged capacitor 126 tocharge to the state of the SRAM cell 102. For example, when the backupsignal 128 is a logical one, and the state of the SRAM cell 102 at theCMP is a logical one, the discharged capacitor 126 may charge to alogical one. When the backup signal 128 is a logical zero, or “off”after the capacitor 126 is charged with the logical one state, thebackup transistor 127 may allow the capacitor 126 to retain a charge.The backup signal 128 may be a global backup signal and effect one ormore SRAM cells 102 simultaneously in an embodiment. When the state ofSRAM cell 102 is to be backed up from capacitive backup circuit 125,feedback line 118 should be at logical zero so that charge fromcapacitive 126 is not drained by inverter 114.

The embodiment shown on FIG. 1 is a symmetric configuration where WBLsare connected to the TRU and CMP nodes. In the symmetric configuration,the feedback line 118 may be coordinated with the WWL 120. For example,during normal reads and writes to SRAM cell 102, feedback line 118 wouldbe a logical one and the reads and writes would occur as is done in aconventional SRAM cell.

In some embodiments, the SRAM cell 102 may have only one WBL, e.g., WBLCMP 112, in single-ended write configuration. In the shownconfiguration, the SRAM cell 102 is in an asymmetric configuration. Whenin the asymmetric configuration, the inverter 116 is the weaker inverterwhile the inverter 114 is the stronger inverter. The stronger inverter114 may be made, for example, by increasing the width of the wires ofthe PFET and NFET. The inverter 114 may also be made stronger byincreasing the size of the PFET and the NFET, according to variousembodiments. The signal output from inverter 114 needs to have astronger drive strength than the inverter 116 because the output fromthe inverter 114 has to overcome the resistance from the feedbacktransistor 117 and the backup transistor 127.

In an embodiment, the inverter 114 may be weaker or stronger than theWBL driver 113 so that the WBL driver 113 can write the capacitivebackup circuit 125. In one embodiment, the inverter 114 may be strongerthan the WBL driver 113 and work in conjunction with the feedbacktransistor 117. The feedback transistor 117 may control the strongercurrent from the inverter 114 to allow the WBL driver 113 to write theSRAM cell 102. In certain embodiments, the inverter 114 may be weakerthan the WBL driver 113, e.g., when the SRAM device 100 performs boththe read and write operations through the WBL 112.

The beta ratio for inverter 116 may be biased to make inverter 116 morereactive to small voltage changes at the CMP node. Furthermore, thesignal at the CMP node can be set strong enough to overpower thecapacitor 126 during charging.

FIG. 2 illustrates a high-level schematic of an SRAM device 200,according to various embodiments. In the shown configuration, the SRAMdevice 200 has the same components of the SRAM device 100 of FIG. 1. TheSRAM device 200 shows four SRAM cells 102 in two rows, a first and asecond row; and two columns, a first and a second column. The firstcolumn is driven by the WBL driver 113 through the WBL 110. The firstcolumn also has a RBL 122 that reads the state of the SRAM cell 102 andoutputs to output 0. As mentioned above, the RBL 122 may be pre-charged.The second column may be driven by the WBL driver 210 through the WBLM−1. The second column has a RBL M−1 that reads the state of the SRAMcell 102 and outputs to output M−1. It is understood that any number ofcolumns between the first column and the second column may exist.

The first row may have a WWL 120, and a FL 118. As described above, theFL 118 may be separate from the WWL 120. The signal to initialize theread operation may be controlled by RWL 123. The second row may have WWLN−1, FL N−1, and RWL N−1. It is understood that any number of rowsbetween the first row and the second row may exist.

FIG. 3 illustrates a timing diagram 300 of the backup operation of theSRAM device, according to various embodiments. Throughout thisdisclosure, the backup operation may be referred to as a backup orbacking-up of the SRAM cell 102. The capacitor 126 may store a memorystate from the SRAM cell 102 in the backup operation. In the backupoperation, the capacitor 126 may automatically back up the memory statefrom the SRAM cell 102 for a set number of cycles. For example, the SRAMcell 102 may automatically backup every 100 cycles, or any number ofcycles as set by a user. However, there may be a trade off with thenumber of backups between cycles and the overall performance of acomputing system. For example, a backup every two cycles would allowmore recent data but may require more overhead in order to charge thecapacitor 126. Both the saving and using data from the capacitor 126 maybe performed for one or more SRAM cells such as in an arrayconfiguration.

The timing diagrams for the backup signal 128, the WWL 120, the feedbackline 118, the TRU, the CMP, the WBL 112, and the capacitor 126 are shownat times T0 through T3. A signal is driven through the WBL 112 from theWBL driver 113. The WBL 112 produces a logical one signal through T3.The WBL 112 signal is transmitted through NFET 121 when WWL 120 is high.The timing diagram 300 begins at time T0. The SRAM cell 102 has alogical one signal at the CMP which is inverted by the inverter 116 toproduce a logical zero signal at TRU.

In certain embodiments, the operation from T0 through T1 may be aninitialization operation for the SRAM device 100, e.g., during a startup phase. For purposes of illustration, it is assumed that the initialmemory state of the SRAM cell is a logical zero (at the TRU) at theinitialization. At a point between T0 and T1, a logical one at thefeedback line 118 is applied to the gate of the feedback transistor 117.The logical one signal may activate the feedback transistor 117 andconnect the output from inverter 114 to the CMP. Once the feedback line118 is turned on, then the SRAM cell 102 may form a feedback circuit.

The timing signals between T1 and T2 may represent the charging of thebackup circuit. At T1, the backup transistor 127 may receive a logicalone backup signal 128 at its gate. The logical one signal causes thebackup transistor 127 to connect the CMP to the capacitor 126. When thebackup transistor 127 receives a backup signal 128 with a logical high,then the capacitor 126 absorbs the state in the SRAM cell 102. Thecapacitor 126 slowly absorbs the charge of the SRAM cell 102. Theabsorption, or charge sharing, causes a momentary drop in the CMPvoltage (and gain in the TRU voltage). The charge sharing may be nearlyinstantaneous and may function as the capacitor 126 charges. The chargesharing may be less significant due to the high resistance of thecapacitor 126 in some embodiments. The charge sharing may be minimal dueto the WWL 120 being turned off. The loss of current at CMP isreplenished with the current from the inverter 114. In an embodiment,the inverter 114 may be stronger than the WBL driver 113, therefore thepass transistor 121 may prevent the inverter 114 from overpowering theWBL driver 113. In some embodiments, the capacitor 126 may need to bedischarged or refreshed prior to writing to erase any of the priorstate.

At a point between T1 and T2, the CMP voltage is constant because thedraw from the capacitor 126 is minimal since the capacitor 126 hasalmost reached its steady-state charge level. The CMP and TRU retaintheir state because the SRAM cell 102 is in a charged state with alogical one signal at CMP which is inverted by inverter 116. In someembodiments, the capacitor 126 may perform a buffering action whichallows the CMP to maintain its state without an external write from theWBL 112. For example, if the backup transistor 127 is turned on whilethe capacitor 126 is charging, then the lack of a write signal from theWBL 112 may cause the capacitor 126 to discharge to the prior write. AtT2, the backup 127 is deactivated with a logical zero backup signal 128which causes the capacitor 126 to retain the charge. At T1, thecapacitor 126 may be overwritten without a refresh, according to variousembodiments. The time may vary depending on the type of signaloverwriting the prior state of the capacitor 126. For example, if alogical zero or low state is overwriting an existing logical one stateon the capacitor 126, then the operation may take longer thanoverwriting a logical zero with a logical one.

FIG. 4 illustrates a timing diagram 400 of the normal write operationfor the SRAM device, according to various embodiments. In the normalwrite operation for the SRAM cell 102, the backup signal 128 is turnedoff and the capacitor 126 retains the memory state from the backupoperation. In the shown example, the WBL driver 113 may write a logicalone memory state to SRAM cell 102 a logical zero memory state asmeasured at the TRU. The timing diagram 400 may begin at T3 and end atT6. At T3, the WBL 112 may shift from transmitting a logical one to alogical zero signal. Between T3 and T4, the WWL 120 may be activated.Once the WWL 120 is turned on, then the WBL 112 may transmit a logicalzero signal to the SRAM cell 102.

At T4, the feedback line 118 may be cycled from a logical one to alogical zero. The switching off of the feedback may be necessary in aconfiguration where the inverter 114 is stronger than the WBL driver113. When the inverter 114 is stronger, then the current from inverter114 may corrupt the signal along the WBL 112. If the feedback 118 isturned off after the WWL 120 is turned on, then the WBL 112 may writethe SRAM cell 102 before the inverter 114 starts to overpower the WBLdriver 113. After the feedback 118 is turned off, then the CMP and TRUmay begin to switch. The feedback 118 may be turned on at some pointafter CMP and TRU switch in order to maintain the memory state of theSRAM cell 102. The time that the feedback 118 is switched off and thenback on may be referred to as a pulse with a pulse width 704. The pulseis described below in FIG. 7A.

At T5, the WWL 120 may be turned off which may cut off the signal fromWBL 112. At T5, the feedback signal 118 may also be at a logical one.The voltage at the capacitor 126 may drop slightly because of theleakage effect of current. The TRU and CMP nodes hold their memorystate. Even though only the normal write operation is shown, the normalread operation may work in a manner consistent with that normally foundin the art.

The time between T3 and T6 is shown as one cycle but may be 100 cycles.The number of cycles may be determined by the dissipation time on thecapacitor 126. The dissipation time on the capacitor 126 is governed bycharge loss. The dissipation time should be greater than the refreshtime. For example, if the capacitor 126 dissipates to ground every 300cycles, then the capacitor 126 may backup the SRAM cell 102 every 100cycles.

In the shown embodiment, the refresh time is fixed, e.g., refreshing thecapacitor 126 every 100 cycles or every 1000 cycles. In someembodiments, it may be possible for the refresh time to be variable. Forexample, the capacitor 126 may be refreshed 100 cycles, then 150 cycles,then 110 cycles. In a variable refresh time, the memory controller mayhave a clock that tracks the number of cycles that have elapsed, e.g.,such as in eDRAM, so that the memory controller may keep track of thepoint of time when the data was read.

FIG. 7A illustrates a circuit 700 that produces a pulse for the normalwrite operation and corresponding waveform diagram 708 produced by thecircuit, according to various embodiments. The circuit 700 may couple tothe WWL 120 and output to 702. The circuit 700 may have a pulsegenerator 710 and a delay circuit 712 arranged in a serial fashion. Thepulse generator 710 and delay circuit 712 may be wired to produce adelayed pulse, e.g., the pulse in the feedback line 118 in FIG. 4. Thepulse signal 701 may result from the pulse generator 710 and a delayedpulse signal 702 may result from the delay circuit 712.

Waveform diagram 708 illustrates the signals that originate from thecircuit 700. WWL 120 may produce a signal from which the pulse generator710 generates a pulse signal 701. The pulse signal 701 may be of aparticular pulse width 703. The delay circuit 712 may take the pulsesignal 701 and produce a delayed pulse signal 702 of a particular pulsewidth 704. The pulse signal 702 may be delayed using any amount of time.According to various embodiments, the pulse width 703 may not matchpulse width 704. The delayed pulse signal 702 may continue to a logicelement such as a NOR gate shown in FIG. 7C (discussed below). The pulsewidth 704 may further match the pulse width between T4 and T5 on thefeedback line 118 in FIG. 4.

FIG. 5 illustrates a timing diagram 500 of the restore operation of theSRAM device, according to various embodiments. The restore operation mayread a stored memory state from the backup circuit 125 and use thestored memory state to write the SRAM device 100. The timing diagram maybegin at T6 and end at T8.

Between T6 and T7, The capacitor 126 would lose some charge in theprocess as noted by the drop in voltage, e.g., due to leakage current orother non-idealities. The feedback line 118 may be turned off, in orderto reduce the effect of the inverter 114. The feedback line 118 may beturned off and on with a pulse. The pulse may have a pulse width 728(described further below). In an embodiment, the inverter 114 is drivinga strong current that would overpower the current from the capacitor126.

Between T7 and T8, the backup transistor 127 is activated which allowsthe high backup signal 128 held by the capacitor 126 to drive the cellsince the CMP node has a low signal. The backup transistor 127 may beactivated with a pulse of pulse width 729 (described below). The CMPnode would be switched from a low to a high signal which would bring theTRU node to a low signal. Once the capacitor 126 is read once, then thecapacitor 126 may not be read again and may need to be written again inanother cycle. In another embodiment, once the capacitor 126 is readthen there may be an active discharge state to pull the capacitor 126toward ground so that it can be written again.

At T8, the backup transistor 127 may be deactivated with a logical lowbackup signal 128 at the control gate. The capacitor 126 may continue tolose charge (e.g., due to current leakage). The feedback transistor 117may be activated at T8, which would restore the SRAM cell 102 to thememory state before the backup.

FIG. 7B illustrates a circuit 749 that produces the pulse for therestore operation for the feedback line 118 and the backup transistordescribed in FIG. 5 and the corresponding waveform diagram 750,according to various embodiments. The circuit 749 may receive an inputfrom an enable signal 732. The enable signal 732 may initiate therestore operation or the backup operation. The waveform diagram 750illustrates the restore operation. The enable signal 732 is shown as aglobal signal but other configurations are contemplated.

The circuit 749 may receive the enable signal 732 at the pulse generator720 and pulse generator 722. As described in FIG. 7A, a pulse generatormay be a configuration of logic elements that produce a pulse of aparticular interval. For example, the pulse generator 720 may beconfigured to produce a pulse of pulse width 728.

The pulse generator 720 may produce a pulse signal 726 of pulse width728 which may further be received by a NOR gate (described below) andtransmitted to the feedback line 118. The pulse generator 722 mayproduce a pulse signal 730. The pulse generator 722 may couple to adelay circuit 724. The delay circuit 724 may delay the pulse signal 730from the pulse generator 722 and produce a delayed pulse signal with apulse width 729 which may also be referred to as the backup signal 128.

FIG. 7C illustrates a circuit 760 that receives an input signal 702 fromthe delay circuit 712 and an input signal 726 from the pulse generator720, according to various embodiments. The circuit 760 may be configuredso that when one input signal, either 702 or 726, is turned on, then theoutput is turned off. In the shown embodiment, the circuit 760 is a NORgate 760 but other configurations are contemplated. The circuit 760 maycouple to the feedback line 118.

FIG. 6 illustrates multiple design structures 600 including an inputdesign structure 620 that is preferably processed by a design process.Design structure 620 may be a logical simulation design structuregenerated and processed by design process 610 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 620 may alternatively include data or program instructionsthat, when processed by design process 610, generate a functionalrepresentation of the physical structure of a hardware device. Whetherrepresenting functional or structural design features, design structure620 may be generated using electronic computer-aided design, such asthat implemented by a core developer/designer. When encoded on amachine-readable data transmission, gate array, or storage medium,design structure 620 may be accessed and processed by one or morehardware or software modules within design process 610 to simulate orotherwise functionally represent an electronic component, circuit,electronic or logic module, apparatus, device, or system such as thoseshown in FIGS. 1 and 2. As such, design structure 620 may include filesor other data structures including human or machine-readable sourcecode, compiled structures, and computer-executable code structures that,when processed by a design or simulation data processing system,functionally simulate or otherwise represent circuits or other levels ofhardware logic design. Such data structures may includehardware-description language design entities or other data structuresconforming to or compatible with lower-level HDL design languages suchas Verilog and VHDL, or higher level design languages such as C or C++.

Design process 610 preferably employs and incorporates hardware orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures shown in FIGS. 1 and 2 to generate aNetlist 680 which may contain design structures such as design structure620. Netlist 680 may comprise, for example, compiled or otherwiseprocessed data structures representing a list of wires, discretecomponents, logic gates, control circuits, I/O devices, models, etc.that describe the connections to other elements and circuits in anintegrated circuit design. Netlist 680 may be synthesized using aniterative process in which Netlist 680 is resynthesized one or moretimes depending on design specifications and parameters for the device.As with other design structure types described herein, Netlist 680 maybe recorded on a machine-readable data storage medium or programmed intoa programmable gate array. The medium may be a non-volatile storagemedium such as a magnetic or optical disk drive, a programmable gatearray, a compact flash, or other flash memory. Additionally, the mediummay be a system or cache memory, buffer space, or electrically oroptically conductive devices and materials on which data packets may betransmitted and intermediately stored via the internet, or othersuitable networking means.

Design process 610 may include hardware and software modules forprocessing a variety of input data structure types including Netlist680. Such data structure types may reside, for example, within libraryelements 630 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 90 nm, etc.). The data structure types may further include designspecifications 640, characterization data 650, verification data 660,design rules 650, and test data files 685 which may include input testpatterns, output test results, and other testing information. Designprocess 610 may further include, for example, standard mechanical designprocesses such as stress analysis, thermal analysis, mechanical eventsimulation, process simulation for operations such as casting, molding,and die press forming, etc. One of ordinary skill in the art ofmechanical design can appreciate the extent of possible mechanicaldesign tools and applications used in design process 610, withoutdeviating from the scope and spirit of the invention. Design process 610may also include modules for performing standard circuit designprocesses such as timing analysis, verification, design rule checking,place and route operations, etc.

Design process 610 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 620 together with some or all of the depictedsupporting data structures, along with any additional mechanical designor data, to generate a second design structure 690. Design structure 690resides on a storage medium or programmable gate array in a data formatused for the exchange of data of mechanical devices and structures(e.g., information stored on an IGES, DXF, Parasolid XT, JT, DRG, or anyother suitable format for storing or rendering such mechanical designstructures). Similar to design structure 620, design structure 690preferably comprises one or more files, data structures, or othercomputer-encoded data or instructions that reside on transmission ordata storage media and that, when processed by an ECAD system, generatea logically or otherwise functionally equivalent form of one or more ofthe embodiments of the invention shown in FIG. 1. In one embodiment,design structure 690 may comprise a compiled, executable HDL simulationmodel that functionally simulates the devices shown in FIGS. 1 and 2.

Design structure 690 may also employ a data format used for the exchangeof layout data of integrated circuits and/or symbolic data format (e.g.,information stored in a GDSII, GL1, OASIS, map files, or any othersuitable format for storing such design data structures). Designstructure 690 may comprise information such as symbolic data, map files,test data files, design content files, manufacturing data, layoutparameters, wires, levels of metal, vias, shapes, data for routingthrough the manufacturing line, and any other data required by amanufacturer or other designer/developer to produce a device orstructure as described above and shown in FIGS. 1 and 2. Designstructure 690 may then proceed to a state 695 where, for example, designstructure 690 proceeds to tape-out, is released to manufacturing, isreleased to a mask house, is sent to another design house, is sent backto the customer, etc.

Although the present invention has been described in terms of specificembodiments, it is anticipated that alterations and modificationsthereof may become apparent to those skilled in the art. Therefore, itis intended that the following claims be interpreted as covering allsuch alterations and modifications as fall within the true spirit andscope of the invention

What is claimed is:
 1. A method of backing up a Static Random AccessMemory (SRAM) cell on an SRAM device to a capacitive backup circuit ofthe SRAM cell, the method comprising: backing up the data written to theSRAM cell by: electrically coupling an input node of cross-coupledinverters to a capacitor of the capacitive backup circuit by turning onbackup switch configured in a series connection between the capacitorand a feedback switch, wherein the capacitive backup circuit is not in afeedback circuit formed by a first CMOS inverter and a second CMOSinverter; and restoring the data stored in the capacitive backup circuitto the SRAM cell by: deactivating an input voltage to the SRAM cell,deactivating the feedback switch, wherein the feedback switch is in thefeedback circuit and different than the backup switch, and electricallycoupling the capacitive backup circuit to the input node in response todeactivating the feedback switch.
 2. The method of claim 1, furthercomprising writing data to the SRAM cell by applying an input voltage toset the input node of cross-coupled inverters to a memory state.
 3. Themethod of claim 2, wherein writing data includes turning off a feedbackswitch to break cross-coupling of the cross-coupled inverters.
 4. Themethod of claim 1, wherein the backing up the data includes storing datafrom the cross-coupled inverters at a time later than a time the datawas backed up.
 5. The method of claim 1, further comprising retainingthe data in the capacitor of the capacitive backup circuit by turningoff the backup switch on the capacitive backup circuit.